The dispensing of small volume reagent droplets with high accuracy represents an important process step in the production of medical diagnostic assays which are frequently based on using two-dimensional arrays of open wells, such as microtiter plates. Some examples of such dispense systems in use are described in the following references: U.S. Pat. No. 4,107,658, U.S. Pat. No. 4,196,615, U.S. Pat. No. 4,417,473, U.S. Pat. No. 4,818,492, U.S. Pat. No. 5,304,347, U.S. Pat. No. 5,601,980, U.S. Pat. No. 6,029,896, U.S. Pat. No. 6,148,666, U.S. Pat. No. 6,213,354, U.S. Pat. No. 6,551,558, U.S. Pat. No. 6,823,730, U.S. Pat. No. 6,851,778, U.S. Pat. No. 6,875,404, US 2001/0016177 A1, WO 98/09151, WO 00/51736, WO 01/89694 A1, WO 02/26499 A1, WO 03/106936, EP 0,164,679, EP 0,355,791, EP 0,505,004, EP 0,725,267, JP 2004251818 A, JP 2006058188 A.
However, the accurate determination of volume actually dispensed into each of the individual wells in an array still remains a problem.
In needle-based dispensing systems, the targeted amount of liquid leaving the inner needle space is, in many cases, very well controlled, e.g. by the specific motion of a dispensing piston, by utilizing specialized pumps with accurate dosing capability, or by application of piezo-electric devices producing a defined volume displacement. However, the amount of liquid actually reaching the receiving well may vary, because part of the liquid leaving the inner needle space is creeping along the outer diameter of the dispensed needle, therefore forming a small amount of liquid that is lost from the particular dispense act. This mechanism may repeat itself in one or more successive dispense acts and a substantial amount of liquid may, consequently, accumulate on the outer needle diameter. Once a critical amount of liquid accumulates, this liquid will join a dispensed droplet, generating an actual dispensed volume that significantly exceeds the targeted dispensed volume.
In certain dispensing arrangements, where the dispense mechanism cannot generate adequate shear force to cleave a droplet, the dispensed droplet is not ejected freely into the air space, but is brought into contact with the bottom of the receiving well, while still in contact with the dispensed needle. This mode of dispensing, which is called touch-off mode, offers a chance for obtaining a very even distribution of dispensed liquid on the well bottom, in particular for droplet volumes below 10 μL. The touch-off dispensing mode may also promote a certain amount of liquid collecting and creeping upwards on the outer diameter of the dispensed needle if optimum touch off conditions are not maintained.
When sample liquid is aspirated into a dispense needle, the needle has to contact the liquid in the supply reservoir. When needle is withdrawn from the reservoir after the aspiration step, residual liquid from the supply reservoir may remain on the outside of the dispense needle. It is also likely that such liquid may join the dispensed liquid in a dispensing step that follows the aspiration step, creating a droplet of incorrect volume.
Therefore an apparatus and method is required for an accurate determination of the liquid reagent volume actually dispensed into each individual well in a two-dimensional open well array.